Dendritic cells (DCs) are antigen-presenting cells (APCs) present in peripheral blood, skin, lymphatic organs, and thymus, and are widely distributed in lymphatic and non-lymphatic tissues (see Steinman, R. M. Ann. Rev. Immunol. 9:271 (1991); Banchereau, J. B. and R. M. Steinman, Nature 392:245 (1998)). Dendritic cells have strong antigen-presenting ability and express antigenic peptides on MHC class I and II on the dendritic cell surface, which activate CD4 and CD8 T cells, respectively. Through this activation, they induce an in vivo immune response against specific antigens (e.g., antigens of pathogenic microorganisms, tumor-related antigens, and transplantation antigens).
The strong ability of DC to induce immunity is useful in immunotherapy (DC therapy) against many tumors. The present inventors have previously demonstrated that DCs stimulated with Sendai virus (SeV) have a strong anti-tumor effect in mice (S. Shibata et al., J. Immunol, 177: 3564-3576 (2006); Yoneyama, Y. et al., Biochem. Biophys. Res. Commun., 355:129-135 (2007)). The anti-tumor effect depends on the number of inoculated DCs. Clinically, the number of inoculated DCs is also thought to have a great influence on the therapeutic effect. However, there may be many cases where only a limited number of DC precursor cells (DC progenitors) can be collected due to the patient's condition. As a result, there is a possibility that the therapeutic effect may become insufficient due to insufficient number of DCs obtained. Thus, there is a demand for methods that efficiently expand limited DC precursor cells.    [Non-Patent Document 1] Steinman, R. M., 1991, Ann. Rev. Immunol. 9: 271-296.    [Non-Patent Document 2] Banchereau, J. B. and R. M. Steinman, 1998, Nature 392: 245-252.    [Non-Patent Document 3] Shibata, S. et al., J. Immunol, 2006 177: 3564-3576.    [Non-Patent Document 4] Yoneyama, Y. et al., Biochem. Biophys. Res. Commun., 2007, 355:129-135.